A cBN sintered body refers to a body obtained by binding cBN particles to one another with a binder phase mainly composed of TiN, TiC, Co, and Al. Each of the cBN particles is a material having hardness and thermal conductivity next to diamond and having fracture toughness superior to a ceramics material. Therefore, a cBN sintered body containing such cBN particles at a high content is excellent in characteristics such as plastic deformation resistance, toughness, strength, and defect resistance.
A cBN sintered body tool employing the cBN sintered body having such characteristics is excellent in that it is better in chemical stability, lower in affinity with iron, longer in life, and higher in efficiency in working because of its high hardness as a material, than a tool material such as a conventional cemented carbide tool and the like, and it is highly evaluated. Such a cBN sintered body tool of high performance has replaced a conventionally used tool in such applications as cutting of Ni-based and iron-based high-hardness difficult-to-cut materials, applications of plastic working of a punching tool for cold forging, and the like.
Here, “cutting” refers to machining of an article into desired dimension and shape while chips are generated. “Plastic working” refers to application of force to a workpiece to deform the same and formation of the workpiece into a product having prescribed shape and dimension. It should be noted that plastic working is different from cutting in that no chips are generated.
Since the cBN sintered body tool has excellent characteristics as described above, it is advantageous in that sudden defect is less likely in any application of cutting and plastic working and it is extremely suitably employed.
Regarding a conventional cBN sintered body tool, for example, each of PTL 1 and PTL 2 discloses a technique of improving hardness and toughness of the cBN sintered body by reducing inclusion of impurities as much as possible assuming that a metal such as Al, oxygen, and the like contained in the cBN sintered body are the impurities, i.e., by increasing a mixing ratio of cBN particles (Japanese Patent Laying-Open No. 07-291732 (PTL 1) and Japanese Patent Laying-Open No. 10-158065 (PTL 2)).
In addition, a cBN sintered body tool has been considered and commonly believed to be high in performance if it has high hardness and high toughness as well as high thermal conductivity. In accordance with this common belief, Japanese Patent Laying-Open No. 2005-187260 (PTL 3) and International Publication No. WO 2005/066381 (PTL 4) each have proposed a cBN sintered body tool in which thermal conductivity, in addition to hardness and toughness, is improved by use of a cBN sintered body containing high-purity cBN particles, which have high thermal conductivity, at a high concentration. Defect of such a cBN sintered body tool is less likely both in a case of plastic working of a material of low ductility and in a case of cutting of a material having high hardness, advantageously.
According to each of the techniques disclosed in PTLs 1-4 described above, the hardness, the toughness, the thermal conductivity, and the like of the cBN sintered body can be improved in performance, which tends to result in less defect while improving wear resistance of the cBN sintered body.
Meanwhile, the cBN sintered body tool is generally structured to have the cBN sintered body joined to a tool base material via a joining layer such that the cBN sintered body is located in its surface for working on a workpiece. The improvement of the performance of the cBN sintered body as in each of PTLs 1-4 raises a new problem other than the performance of the cBN sintered body as follows: insufficient adhesion between the cBN sintered body and the tool base material causes the cBN sintered body to fall from the tool base material during working, with the result that the tool becomes unusable.
To address this, in each of Japanese Patent Laying-Open No. 2007-276079 (PTL 5) and Japanese Patent Laying-Open No. 11-320218 (PTL 6), it is considered to revise the composition of the joining layer in order to improve the adhesion between the cBN sintered body and the tool base material. Specifically, in PTL 5, for the composition of the joining layer, not less than 10 mass % and not more than 30 mass % of Cu, not less than 2 mass % and not more than 10 mass % of Ti, and not less than 1 mass % and not more than 4 mass % of Ni are employed with a remainder thereof containing Ag and an inevitable impurity, for example. The use of such a material leads to improved adhesion of the joining layer to the cBN sintered body, thereby improving joining force between the cBN sintered body and the joining layer.